(1) Field of the Invention
The general technical field of the invention is fabricating parts made of composite material and in particular parts that include a reinforcing coating. Such parts are often desirable in technical applications and in particular in aviation applications, for example for airplanes, helicopters, or other aircraft.
The present invention relates more particularly to fabricating a reinforcing coating that uses “fiber-placement” technology.
(2) Description of Related Art
The term “fiber-placement” is used herein to cover both the technology that uses a winding mandrel associated with a laying arm, and also the technology of placing fibers by means of a robot arm that performs laying only.
Fiber-placement is a technology for laying fibers, slivers, rovings, or other similar reinforcing strands, whether dry or pre-impregnated with resin, on preferably convex or plane surfaces of a three-dimensional body to be covered. Such placement of reinforcing fibers on a solid surface of the body for covering is performed by applying and positioning slivers, rovings, or other similar reinforcing strands by means of a positioning device or placement head that includes at least one system for guiding fibers and applying pressure to them on the surface, such as a flexible roller fitted with a latex skid for guiding the slivers. The placement device thus serves to position the fibers on the surface by direct mechanical contact without any need for particular tension in the fibers. The placement head is itself driven by a drive system such as a robot arm or a positioning gantry, having its degree of freedom servo-controlled to match the shape of the surface of the body for covering.
The fiber-placement method thus makes it possible, in particular, to cover concave surfaces within the limit set by the size of the head, and also enables discontinuous fiber-laying operations to be performed automatically such as operations of starting, laying, or cutting reinforcement. The paths given to the fibers being laid may be very complex, but they remain limited in particular by the apparent stiffness in the plane of the sliver of the material being laid. A fiber-placement installation may be designed to have a plurality of placement heads that work simultaneously on a single part, in particular for the purpose of improving the productivity of the system.
Winding is performed on a solid body, preferably a body of revolution, and it is applied by performing helical type relative movements between the part for covering and the spool or reel of material for laying, after initially connecting one end of the fibers on the part for covering. The helical type movement is provided by a combination of at least one movement in rotation and one movement in translation, typically along an axis parallel to the axis of rotation. Additional other degrees of freedom of movement in translation or rotation are possible, depending on the complexity of the shapes for covering.
The reinforcing material is thus laid continuously and it is positioned and applied solely under the effect of tension that is controlled between the point of laying on the surface and the exit point from the system for supporting the spool of reinforcing material. This tension, and the location of the outlet point, typically constituted by an eyelet, are determined by controlling the winding machine to comply with the characteristics of the reinforcement that is to be made. The reinforcing material is thus not positioned by being applied to the surface by means of a roller or some equivalent element.
The above-described “simple” winding may also present variants that are “complex”, in particular for the purpose of improving the productivity of the operation. For example, it is possible to increase the number of slivers that are wound simultaneously on the part, either by associating them with one another to form a wider or thicker sliver, or by distributing them over a plurality of locations on the part in a longitudinal direction, i.e. parallel to the axis of rotation or by distributing them in a circular manner, i.e. in a ring around the rotating part.
The braiding method, another method of laying that is somewhat similar, differs from the winding method by the fact that two groups of slivers or rovings are laid simultaneously on a solid mandrel following two groups of coaxial helical paths in opposite directions, crossing in alternation over and under one another so as to lay the reinforcement in a manner that is comparable to closed and “tubular” weaving.
The invention is thus applicable in the field of making composite-material parts by placing fibers, e.g. for the purpose of making a coating on a helicopter blade.
In the context of the present invention, the following definitions should also be taken into consideration.
The term “laying” is used to cover an operation of placing one or more slivers of composite material of a given layer on a part.
The term “layer” means a partial or total covering of the reinforcing material on a given surface of the part in a thickness that is equal to the unit thickness of the laid material, typically fibers, slivers, rovings, or other equivalent strands.
The term “number of laying operations” corresponds to the number of laying operations needed to place all of the slivers of a layer on a part.
The term “pattern” should be understood as the visible design obtained by one or more crossing lines of slivers at the surface of the coating or in the thickness of the coating, in particular when they are covered by a surface fabric, e.g. in order to facilitate a paint finishing operation.
By definition, the term “pitch” is a laying empty space between two slivers laid side by side, either consecutively or simultaneously depending on the method, and in the same direction, as measured along a direction perpendicular to the sliver laying direction, with the convention that a pitch of n corresponds to the empty space left between two slivers being equal to (n−1) times the width of the sliver.
The term “pre-impregnated sliver” should be understood as having fibers, e.g. in the form of a sliver, that prior to being placed on the part, are impregnated with a resin type matrix.
The making of coatings, e.g. on a helicopter blade, generally consists in stacking pre-impregnated woven fabric that is said to be balanced in the warp and weft directions, the fabric being arranged at ±45° relative to the pitch so as to ensure that twisting moments are taken up. In other words, this orientation serves to optimize the mechanical behavior of the blade in twisting. Such an arrangement of woven fibers presents good behavior in the event of an impact, since damage propagation is limited by the interlacing of the fibers that, by definition, is always present in a fabric that is woven. It should be observed that under such circumstances, additional orientations are needed for taking up centrifugal forces and forces due to the blade bending, e.g. 0°/90°.
The present trend is to use composite preforms that are made by fiber placing in order to improve productivity.
The making of parts by placing fibers, as obtained using placement means of the robot arm or equivalent type, generally leads to surface appearances that are similar to those of superposed unidirectional sheets, since the fiber slivers are arranged edge to edge. Such surface appearance is unsatisfactory relative to ability to withstand damage insofar as damage propagates quickly in the fiber direction.
In the event of an impact on a surface made of composite material of the unidirectional sheet type, it is found that damage is relatively large since breaks (cracks) in the impregnation resin or matrix propagate very easily between the aligned fibers, with this continuing so long as the crack does not encounter a physical obstacle.
Various documents that approach the invention are mentioned below.
The NLR document dated Jun. 16, 2010 “NLR and TU Delft improve composite damage tolerance with new APPLY fiber architecture” describes the use of crossing layers of composite materials constituting placement patterns in order to increase damage tolerance. However the patterns described are not optimized for adapting to the various impacts and strikes to which a helicopter blade or a propeller blade can be exposed.
Document US 2004/0074592 describes a composite material with multiple orientations using an impregnated resin. Tapes of filaments are impregnated with a solid or semisolid resin. The resulting material possesses a varying number of layers and orientations within a sheet of the composite material that is formed. A preform is produced to determine the load characteristics of the composite component, and the number of layers and their orientations are selected as a function of those characteristics. An offset of 90° is provided between the filaments.
Document US 2009/0098337 describes articles such as a turbine cage, that are substantially cylindrical and made of composite material. Depending on the example, several plies of unidirectional fibers are interlaced in various orientations in the range 0° to 90°, including 45°, in certain circumstances.
Document U.S. Pat. No. 6,227,805 in the name of the Applicant describes a technique for making a blade out of composite material using resin transfer molding (RTM) in which a coating for the shell, the spars, and the attachment means of the blade are constituted respectively by at least one cutout of woven fabric that includes at least two crossed plies of crossed carbon fiber woven fabric, said plies being superposed on one another so as to form a predetermined angle between them.
Document EP 1 035 963 describes an applicator and control system serving to create patterns by leaving a gap in a tape, without a clear definition of any type of pattern. One of its objects is to improve the structural performance of a composite part. The limitation associated with the fiber-placement means of the type comprising solely a robot arm, as is used for making the coatings in a female mold, is such that the resulting draping can be considered as comprising superpositions of unidirectional sheets, and therefore does not have behavior that is optimized in terms of damage tolerance.